"our people are core to our success"
Indeed they are, and you have just demonstrated that there are several layers to your core, meaning some people are more core than others.
Engineers are already, and logically, the best-paid employees in a company. Giving everyone a fixed percentage of bonus means that engineers will get the most.
Engineers will agree with that.
But fiddling around with the bonus level following what department you are in just demonstrates the true importance you give to said department. That means that IT is valued at half the engineering department at Arm. I can understand that from a beancounter point of view, but the truth is : if engineers stop working, you can still make and sell what you've got. If IT stops working, you're not making or selling anything any more.
So who's more important ? Discuss.